The second, or vital force, is something superadded to the inherent sort. It is an _influxus_, derived to any fibre or set of fibres, from that greater sum of force, which arises out of a more elaborate, complex, and exalted organization. It varies in proportion as the vital spirits flow with more or less freedom; and in proportion as their quality is more or less perfect. The third kind, or robur animale, may be supposed to depend on the organic constitution of the brain and nerves, and varies according to the state of that organization. We cannot help adverting to the resemblance between these two latter kinds, and the contractilite organique, and contractilite animale, of BICHAT; and this robur comprises, as we shall show hereafter, both the contractilite and sensibilite of the French physiologist. GLISSON, in his chapter de Irritabilitate fibrarum, commences by remarking that a motive faculty existing in any fibre, unless it were of an irritable nature, would leave such fibre in one of the two following states: 1. It would either never cease from action, or 2ndly, being once at rest, its motion could never be reproduced; but the varieties and differences which we see in the actions of fibres, clearly demonstrate them to be possessed of irritability: i.e. if a fibre may be by turns in a state of action and repose, it is evidently possessed of a quality, whereby it can be induced to move if in a state of rest; this quality he terms irritable, or irritability. The next inference from this power of alternate activity and repose is, that the fibre is possessed of a faculty, whereby it can _perceive_ an irritation offered to it; but this perception of irritation further implies an _appetence_ for a change of its actual state, before the motion can really take place. Perception, appetence, and motion, make a triunit. "In the mean time, says he, as sensitive appetence, and sensibility, are frequently confounded with natural perception, in this irritation of the fibres," he divides it into three kinds, viz. Natural Perception, Sensitive Perception, and Perception regulated by animal appetency. Natural Perception is that principle whereby a fibre perceiving any alteration offered to it, whether pleasing or displeasing, is excited either to accept that change, or to avoid it, and moves accordingly. Sensitive Perception, is that kind, in which a fibre, perceiving a change effected in some other organ, is impelled ad aliquid appetendum, and to move conformably. The third sort, or Perception regulated by animal appetency, is that in which the brain directs from within, such movements of the muscular fibres, as are requisite for the execution of any purpose.